This invention relates to franking machines and in particular to electronic circuits for carrying out accounting functions in franking machines
It is known in franking machines to employ electronic microprocessors and memory devices for carrying out accounting functions in relation to franking transactions effected by the machine, the accounting functions including the maintenance of a record of credit available for use in franking mail items and a record of accumulated postage value used in franking mail items. The microprocessor and memory devices are implemented as semi-conductor devices fabricated in the form of semi-conductor integrated circuits. In operation, the microprocessor is controlled by a program to carry out various functions selected by a user of the machine for example by keying command signals on a keyboard. When the franking machine is in normal operational mode for carrying out franking, a user wishing to frank a mail item keys in the value of franking required and then operates one or more keys to cause the microprocessor to carry out a sequence of operations including carrying out accounting functions, setting the printwheels to the value of franking and finally driving the printwheels to effect printing of the franking on the mail item. The accounting functions carried out by the microprocessor include reading the contents of a descending register to check that there is sufficient credit available to cover the value of the required franking, decrementing the value stored in the descending register by an amount equal to the value of the franking, incrementing the value of the contents of an ascending register by an amount equal to the value of the franking, and incrementing by one the count in an item count register. The values stored in the registers, particularly those in the ascending and descending registers, must be maintained with absolute precision because these registers provide the accounting information which is the record of the monetary value of franking issued by the machine and for which payment has been made, in the case of pre-payment, or will be made to the postal authority. In order to maintain the integrity of the registers, each register is usually replicated so that each value is stored in four separate locations. The microprocessor periodically checks the magnitudes of each value stored in the registers and if there is any discrepancy between the magnitudes in the separate locations of any stored value, the microprocessor causes the machine to lock out and prevent further use for franking until it has been checked by a service engineer. The electronic circuits are housed in a secure housing designed to prevent unauthorized access to the circuits whereby the machine could be used fraudulently. In addition it is necessary to ensure that stray electrical signals, such as pulse spikes on the mains power supply, are not picked up by the electronic circuits to cause false values to be stored in the memory devices. Conventionally, accessing of memories utilises parallel data transfer and the address signals for addressing a specified memory location are input in parallel to the memory device. A further signal is utilised to select reading from or writing to the specified location of the memory. Accordingly the microprocessor and memory devices are interconnected for the reading and writing of data by means of a plurality of lines, one group of lines being utilised for carrying address signals to specify the memory location to which data is to be written or from which data is to be read, another group of lines for carrying parallel data signals and a line for carrying control signals from the microprocessor to the memory devices. The address and data lines are actively connected to the memory devices and hence there is a high risk that any stray signal induced on a data line will cause distortion of the data signals and result in corruption of the value stored in one or more of the registers of the memory devices. The microprocessor, memory devices and other electronic components are mounted on a printed circuit board having conductive tracks to provide the required connections between the components. The plurality of lines for carrying data signals and address signals between the microprocessor and the memory devices occupy a significant area of the printed circuit board and this places undesirable constraints on the minimum size of printed circuit board which can be used to mount and interconnect the components.